1. Field of the Invention
The invention relates generally to explosive blasting and more particularly to a system for using the global positioning system (GPS) for detonating shaped explosions.
2. Description of the Prior Art
In blasting operations, it is important to achieve the maximum breakage for a given amount of explosives in a blast. It is further important to minimize the effects of the blasting on nearby structures by reducing the amplitude of ground vibration produced by the blast. The principle method for achieving these objectives is to shape the blast by sequentially timing the detonation of a plurality of explosive charges placed at selected locations within an area of operation. The locations may be separated by several meters and may incorporate up to a thousand or even more explosive charges. An exemplary blasting system might require timing accuracies of a few tens of microseconds. Inaccuracies in the sequential timing or misplacement of any of the charges will degrade the accuracy of the shape of the blast. Similar issues are also important for seismic operations.
Traditionally a blasting system uses a web of electrical wires extending from a central node to detonators located with the explosive charges. The detonators may be triggered sequentially from the central node. However, because the electrical wiring web is likely to be destroyed before the sequence of triggers is completed, it is common to transmit an initial, common trigger to charge controllers that are located with the detonators where the charge controllers have selectable time delays for providing the sequential detonation times to the detonators. It is important to minimize the labor and material costs of the electrical wiring and the detonators because they are used only once and destroyed in the blast. Low cost charge controllers have for many years used short pyrotechnic trains of differing lengths having a fixed burn rate for providing the sequential times. However, this type of charge controller is not entirely satisfactory because the statistical variation in the fixed burn rate for different pyrotechnic trains limits the accuracy of the sequential times that can be achieved, thereby reducing the precision of the shape of the blast. In order to improve this accuracy, recent systems have used electronic time delay circuits in place of the pyrotechnic trains. The accuracy of such electronic time delay circuits depend upon the drift rate of an internal clock and the length of the delay time between the initial trigger and the detonation time. A simple electronic delay circuit can be constructed using a voltage controlled oscillator (VCO) as the internal clock. However, the accuracies of VCO clocks are typically not satisfactory unless they are stabilized. Such stabilization adds complexity and expense to the system. In order to improve the accuracy, crystal oscillator clocks have been used. Unfortunately, the accuracy of low cost crystal oscillators is insufficient for some applications. Further, in practice the use of oscillator clocks has not been entirely satisfactory for operation in harsh vibration environments such as those experienced in the blasting industry.
The routing of the electrical wire web from the central node to the charge controllers is laborious and error prone. Great care must be taken to inspect the wiring and test the connections. Also, the remains of the wires may need to be cleaned up after the blast so that the blasted material is not contaminated. In order to eliminate these problems, radio signal communication systems have been used for triggering the charger controllers. However, the requirement for a transceiver for transmitting and receiving the radio signals increases the cost of the charge controller. Further, the radio signals for such systems require a time consuming firing protocol. The protocol may use redundant signal transmissions and/or several retries to ensure that all the charge controllers have received the initial trigger and that a spurious signal cannot prevent the detonation of a particular explosive charge, or worse yet, cause the charge to be triggered unexpectedly. Unfortunately, the time for implementing the protocol increases the length of time over which the time delay circuits must maintain their accuracy, further increasing the cost of the charge controller. An exemplary protocol may require up to 1.5 seconds per charge control station. For 1000 explosive charges a total time of 1500 seconds would be required to verify that all the charge control stations were operational and in receipt of their detonation times. In order to obtain ten microseconds of detonation timing accuracy after 1500 seconds, the drift rate of the internal clock of the charge control station must be better than about 6.7.times.10.sup.-9. Such drift rate is difficult to obtain without the use of an atomic clock.
There is a need for an inexpensive apparatus and a method for detonating a shaped blast without using an electrical wire web where the accuracy of the sequential timing of explosive charges is independent of the length of time for implementing a firing protocol.